This application is a revised proposal of a competing renewal that initially focused on identifying determinants and mechanisms by which HIV-1 Nef enhances viral infectivity. Published and preliminary data are presented to provide evidence that Nef contains sorting motifs that dysregulate cellular protein sorting. It is hypothesized that this effect underlies Nef's varied functions including the ability to downregulate CD4 and HLA class-I molecules and to enhance viral infectivity and replication. The primary goal of the proposal is to identify and characterize these motifs, elucidate the interactions that they mediate, and determine the biological consequences of these effects. The initial application was criticized largely on the grounds that alternative approaches and confounding variables had not been considered, that potentially interesting differences in SIV Nef had not been incorporated into the hypothesis, and that the proposal suffered at times from an overall lack of clarity. The PI has responded to these criticisms with an extensively rewritten proposal that incorporates several new (albeit risky) approaches.